Comparison of ion-pairing and reversed phase liquid chromatography in determination of sulfamethoxazole and trimethoprim

Two simple, rapid, and sensitive HPLC methods have been developed for the simultaneous determination of sulfamethoxazole and trimethoprim in their pure and dosage forms, one utilizing reversed phase HPLC and the other ion-pair HPLC. In the reversed phase HPLC method (A) the mobile phase consists of 0.05% aqueous solution of formic acid with pH adjusted to 4.50.2 with triethylamine:acetonitrile:tetrahydrofuran 50:49:1 (v/v), and the mobile phase pumped at flow rate of 1.0ml min-1. An Appolo LC18 column (5.0m), 250mm length4.6mm diameter, was utilized as the stationary phase. Detection was affected spectrophotometrically at 254nm. In the ion-pair HPLC method (B) the mobile phase consisted of methanol:buffer 35:65 (v/v) with the buffer composed of potassium dihydrogen phosphate 0.3M and sodium heptan sulfonic acid 5.0mM. To 500ml of buffer was added 2.0ml triethylamine, and then the pH was adjusted to 5.0 with phosphoric acid, and the mobile phase was pumped at a flow rate of 1.2ml min-1. A Hypersil C18 column (5.0m), 150mm length4.6mm diameter, was utilized as the stationary phase. Detection was affected spectrophotometrically at 254nm. Linearity ranges for sulfamethoxazole and trimethoprim were 1.0-110 and 1.5-98g ml-1, respectively, with method A and 0.5-100 and 1.0-125gml-1, respectively, with method (B). Minimum detection limits obtained were 0.1969 and 0.3451gml-1 for sulfamethoxazole and trimethoprim, respectively, with method A, and 0.1377 and 0.2454gml-1 with method (B). The proposed methods were further applied to the analysis of tablets containing the two drugs, and the results were satisfied.